1. Field of the Invention
The invention relates to a filter member for horizontal disc filters with a hub and a base plate connected therewith, on which a filter medium is fastened which forms a support for the deposition filtration and the filtration forming a cake which support extends close to a peripheral edge of the base plate, and with cavities arranged in the base plate and extending in the radial direction at a slant toward the hub.
2. Description of the Related Art
Horizontal disc filters comprise conventionally a plurality of disc-shaped filter members stacked atop one another whose filter surfaces are arranged horizontally. In this connection, the supply can be performed from the interior, i.e., through a central inlet channel, or from the exterior, i.e., from the inner space of the filter container.
From DE 34 05 483 A1 a disc filter member for a horizontal disc filter is known which is suitable for forming a stack of filter members on a central tube. This filter member comprises a hub as well as a base plate that is connected thereto and is of a circular annular shape when viewed in a plan view. The circular annular base plate is connected with its inner radius to the outer periphery of the hub, and a ring-shaped, downwardly formed cavity is provided close to the outer peripheral edge. A support fabric, that rests with its inner edge on a projection of the hub and is supported over a large surface area on the circular ring of the base plate, extends with its outer area past the ring-shaped cavity. A filter fabric is arranged above the support fabric and spans the hub, the support fabric, and the outer peripheral edge of the base plate, and is secured by a clamping ring seal tight at the peripheral edge of the base plate. At the underside of the base plate radially extending cavities are provided which are distributed with a large angular spacing about the circular periphery and which connect the ring-shaped cavity at the outer periphery of the base plate with a collecting channel surrounding the hub. These radial cavities serve to guide away the filtrate, which collects in the ring-shaped cavity, toward the collecting chamber and from there into the central shaft.
The large mesh support and drainage fabric ensures that the fine mesh filter fabric has a certain spacing to the base plate so that passage of liquid is altogether ensured thereby. Moreover, the filtrate is guided radially inwardly, i.e., toward the hub, via the openings formed within the support and drainage fabric. Since the filter fabric during the filter operation loses its tension at least partially, from time to time a detachment of the clamping ring and a re-clamping of the filter fabric as well as a new fixation by means of the clamping ring are required. Should wear of the filter fabric occur, it must be replaced completely. The filter members of the known horizontal disc filter are therefore complex components with respect to their construction and manufacture as a result of their operating principle.
It is an object of the invention to provide a filter member for horizontal disc filters of the aforementioned kind which is simpler in its configuration and significantly more wear resistant.
In accordance with the present invention, the object is solved in that in the base plate a plurality of cavities are provided which extend radially and are positioned closely adjacent to one another, with narrow projections formed therebetween, and that the filter medium is a thin filter plate with very small openings, that is supported on the hub as well as on the projections between the cavities and on a surface on the peripheral edge of the base plate and is at least partially connected thereto by a connection by material bonding in the area of the support surfaces.
The important advantages of the invention are to be seen in that the filter medium, that is, the thin filter plate and the base plate, are comprised of a material combination which allows a connection by material bonding on the inner as well as the outer peripheral edge and a plurality of connecting points of the filter plate with the base plate. The number of individual parts per filter member is reduced as well as the mounting expenditure, while at the same time the manufacturing quality is increased. Also, the removal of the filtrate through the cavities is more favorable because they form clearly defined channels in contrast to a drainage fabric which has a greater through flow resistance. By eliminating the drainage fabric, a source of soiling is eliminated because filter residues can collect in the drainage fabric, primarily in the area close to the outer edge. The filtration aids which have penetrated the filter medium as well as product particles can be transported away without resistance in the filtrate direction in the smooth cavities at an incline. The members can be permanently kept clean so that a stable end constant operational state is ensured. A drift in the filtrate quality resulting from soiling is thus prevented.
For the purpose of cleaning the horizontal disc filter, the filter members are rotated wherein the cavities of the base plate of the member package act as turbulators. The greater turbulence generated by the cavities in the base plate and the thus increased shearing forces enhance the cleaning effect.
According to a preferred embodiment of the invention the filter plate is welded to the support surfaces of the base plate. By welding the filter plate to the base plate along the projections as well as at the inner and outer edge of the base plate, a dimensionally stable sandwich construction results which, because of the plurality of connecting locations and the minimal spacing between the connecting points, ensures an extremely high stability of the construction and is also able to withstand increased forces in a backflow load situation. An inflation of the filter members by increased inner pressure is thus prevented. Expediently, the filter plate as well as the base plate are comprised of stainless steel or a stainless steel alloy so that for most applications the most favorable conditions are provided with respect to mechanical strength as well as chemical resistance. As an alternative to this, the base plate and the filter plate can also be comprised of plastic, preferably PVC, PE, PP, or PVDF.
In the case where the base plate is comprised of a sheet metal, it is preferably a stamped or deepdrawn part in which the cavities are embodied as beads. These beads in the base plate provide an increased dimensional stability so that the base plate, with respect to conventional arrangements, has a reduced thickness while maintaining the same dimensional stability. In this context, the thickness is preferably  less than 6 mm; a thickness of approximately 1.5 mm is considered especially suitable. Due to the minimal thickness the material expenses decrease, and the weight of the filter members is also significantly reduced. Because of the weight savings in this construction the manipulation of the filter members during assembly and demounting of the horizontal disc filter is facilitated. Moreover, when rotating the filter member packages for the purpose of cleaning, a reduced inert mass is to be accelerated so that the energy consumption is lowered and the required driving power is also reduced. The weight reduction thus results in the use of smaller drive devices and a reduced loading of the foundation so that cost savings can also be achieved in this respect.
For the purpose of closing off as few as possible of the openings in the filter plate by resting it on the projections, it is advantageous to design the upper edge of the projections, with respect to its cross-sectional shape, as arc-shaped domes so that a linear contact of the filter plate is provided. In this context it is expedient that the projections are substantially of a constant width and the width of the cavities between these projections varies. The spacing between two projections is maximally approximately 50 mm, preferably, however, less than 15 mm, so that a bending of the relatively thin filter plate into the cavities can be prevented. The cross-section of the beads can be designed differently depending on the geometric conditions of the filter members wherein in this context especially a V-shaped or a U-shaped cross-section is to be considered.
In order to make a complete emptying of the filter members possible, the cavities are designed in their longitudinal direction such that an incline toward the hub is provided which is at least 0.5xc2x0, relative to a horizontal axis, preferably however approximately 2xc2x0. With this measure, no residues remain in the interstitial spaces which is especially important in connection with cleaning by chemicals or when changing the product. The beads with a slant toward the interior allow a safe venting of the sludge space below the base plate. Moreover, with this shape an outwardly open wedge shape of the sludge space below the filter member results so that, even when overshooting the filter, the filter cake cannot wedge itself and can be removed without difficulties.
Since the projections extend in a star shape toward the hub, it is expedient in regard to embodiments in which the projections have a constant width, that first and second projections of different lengths are provided, wherein the first and second projections are arranged alternatingly and distributed about the periphery of the base plate. The projections start respectively at the outer edge of the base plate and thus extend over different lengths in the direction toward the hub. In this connection it is preferred that the length of the first projections is approximately xc2xe to ⅘ of the outer radius of the annular ring and the length of the second projections is approximately xc2xe of the length of the first projections. Depending on the size of the outer radius of the base plate, it is also possible to provide third projections which are significantly shorter than the second projections and which extend respectively between the first and second projection at a spacing thereto. The length of the third projections in this connection can be approximately ⅙ of the length of the first projections.
In order to maintain the spacing of the outer edges of the filter members in the axial direction in a reliable fashion, it is advantageous that several axially extending spacer elements, extending in the axial direction of the member package, are welded onto the peripheral edge of the base plate in a uniform distribution about the circular periphery.